The invention concerns circuitry for reversing field. Also see related U.S. Pat. Nos. 5,229,902 and 5,220,467
Circuitry of this kind is employed in magneto-optical recording and playback equipment for example to reverse the magnetism in the magnetic layer of a magneto-optical recording medium.
Magneto-optical disks are one known form of magneto-optical medium They have a transparent layer with a magneto-optical layer behind it. Information can be stored on and read from the latter. How information is stored on and read from a magneto-optical disk will now be described.
A laser beam focused on the disk heats the magneto-optical layer to a temperature in the vicinity of its Curie point. It is usually sufficient to heat the layer to approximately its compensation point, which is below the Curie point Behind the focal point on the disk is an electromagnet that magnetizes the area heated by the laser beam in one or the other direction. When the laser beam is turned off, the heated area will cool to below the compensation point again, and the magnetic direction established by the electromagnet will be retained, "freezing in." Individual bits are accordingly stored in domains of different magnetic direction, one corresponding to a logical ONE and the other to a logical ZERO.
The data are read out by exploiting the Kerr effect. The plane of polarization of a linearly polarized beam of light is rotated at a measurable angle when the beam if reflected by a magnetized mirror. The plane will be rotated right or left depending on the mirror's direction of magnetization. Since, however, the individual domains on the disk act like magnetized mirrors, the plane of polarization of any beam scanning the disk will be rotated right or left at a measurable angle.
From the rotation of the plane of polarization of the beam of light reflected from the disk, an optical pick-up can determine whether the bit being scanned is a ONE or a ZERO.
One known approach to magnetizing a magneto-optical layer in one direction or the other involves circuitry with a coil behind the magneto-optical disk that acts like an electromagnet. The coil is large enough to reverse the magnetism of the total area scanned by the optical pick-up. This area is, depending on the type of recording and playback equipment for example either a strip that is either radial or in the form of an arc of a circle and extends from the edge to the center of the disk. Since the field must attain a certain minimal strength over the total strip in order to reverse the strip's magnetism, the cross-section and hence the inductivity of the coil must be relatively large.
In another known approach, the coil is secured to the optical pick-up and can for example be wound around the pick-up's objective lens. Since the coil in this version is displaced along with the pick-up along the disk's information-storage tracks by a tracking circuit, a smaller cross-section and lower inductivity will generate an equivalent minimal field strength because it is not a radial or arc-shaped strip but only a small and for example circular area with the almost punctual laser spot as a center in the magneto-optical layer that is being magnetically reversed.